The carburetor was invented by the Hungarian engineer Donat Banki in 1893. Fredrick William Lanchester of Birmingham, England experimented early on with the wick carburetor in cars. In 1896 Frederick and his brother built the first petrol driven car in England, a single cylinder 5 hp (4 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design. This version completed a 1,000 mile (1600 km) tour in 1900 successfully incorporating the carburetor as an important step forward in automotive engineering. The word carburetor comes from the French carburet, meaning ‘carbide’. To carburet means to combine with carbin. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.
The carbureter is a device which mixes air and fuel for an internal-combustion engine. Carburetors are still found in small engines and in older or specialized automobiles. However, most cars built since the early 1980s use computerized electronic fuel injection instead of carburetion. The majority of motorcycles still are carburated due to lower weight and cost. Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some, primarily with greater than 4 cylinders or higher performance engines, use multiple carburetors or multi-choke carburetors (the latter being a number of intakes in a single body). Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never "flooding" the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil covered mesh; this was an effective system in a time when paper air filters did not exist. Today, most automotive carburetors are either downdraft (flow of air is downwards) or side-draft (flow of air is sideways).
The carburetor works on Bernoulli’s principle: the fact that moving air has lower pressure than still air, and that the faster the movement of the air, the lower the pressure. Generally speaking, the throottle or accelerator does not control the flow of liquid fuel. Instead, it controls the amount of air that enters the carburetor. Faster flows of air and more air entering the carburetor draws more fuel into the carburetor due to the partial vaccum that is created. The goal of a carburettor is to mix just the right amount of gasoline with air so that the engine runs properly. If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The carb is in charge of getting the mixture just right.
- A carburettor is essentially a tube.
- There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube. You can see this circular brass plate in pic 1.
- At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created. The venturi is visible in pic 2
- In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel. You can see the jet on the left side of the venturi in pic 2.
The carb is operating "normally" at full throttle. In this case the throttle plate is parallel to the length of the tube, allowing maximum air to flow through the carb. The air flow creates a nice vacuum in the venturi and this vacuum draws in a metered amount of fuel through the jet. You can see a pair of screws on the right top of the carb in photo 1. One of these screws (labelled "Hi" on the case of the chain saw) controls how much fuel flows into the venturi at full throttle. When the engine is idling, the throttle plate is nearly closed (the position of the throttle plate in the photos is the idle position). There is not really enough air flowing through the venturi to create a vacuum. However, on the back side of the throttle plate there is a lot of vacuum (because the throttle plate is restricting the airflow). If a tiny hole is drilled into the side of the carb's tube just behind the throttle plate, fuel can be drawn into the tube by the throttle vacuum. This tiny hole is called the idle jet. The other screw of the pair seen in photo 1 is labelled "Lo" and it controls the amount of fuel that flows through the idle jet.
Both the Hi and Lo screws are simply needle valves. By turning them you allow more or less fuel to flow past the needle. When you adjust them you are directly controlling how much fuel flows through the idle jet and the main jet. When the engine is cold and you try to start it with the pull cord (choke in layman terms), the engine is running at an extremely low RPM. It is also cold, so it needs a very rich mixture to start. This is where the choke plate comes in. When activated, the choke plate completely covers the venturi. If the throttle is wide open and the venturi is covered, the engine's vacuum draws a lot of fuel through the main jet and the idle jet (since the end of the carb's tube is completely covered, all of the engine's vacuum goes into pulling fuel through the jets). Usually this very rich mixture will allow the engine to fire once or twice, or to run very slowly. If you then open the choke plate the engine will start running normally.
This is just the basics. If you want to know more please feel free to mail me. Along with data, I have couple of videos showing the working of a carb and choke plate (with audio explanation), which I can mail to anyone who is interested. This will make it easier for you to visualise the process. Hope this article aided in your understanding of an automobile engine, that would be mission accomplished for me. For more technical articles watch this space (maybe some of you could mail me any particular topic that you want to know about, I’ll be happy to help out).
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